This is a pilot clinical trial to assess whether the administration of diltiazem may be able to decrease the development or progression of hypertrophic cardiomyopathy (HCM). Diltiazem is a commonly used medication for the treatment of high blood pressure and studies on animals with HCM suggest that diltiazem decreases disease development. This study specifically targets individuals in the "prehypertrophic" phase of HCM-- those with documented sarcomere gene mutations without echocardiographic or EKG evidence of LVH, and therefore without a clinical diagnosis of HCM.